Insights into the 2023 ACR/EULAR antiphospholipid syndrome classification criteria: findings from a cohort of 205 patients with primary APS

Objectives The 2023 American College of Rheumatology / European Alliance of Associations for Rheumatology (ACR/EULAR) antiphospholipid syndrome (APS) classification criteria distinguish between anticardiolipin (aCL) or anti-beta 2-glycoprotein I (a beta 2GPI) IgG vs IgM isotypes, having isolate IgM positivity a low weight, insufficient for APS classification, and define aCL and a beta 2GPI thresholds based on fixed cut-off values. We aimed to assess the performance of the 2023 ACR/EULAR criteria in a cohort of primary vascular APS patients (PAPS), previously classified according to the Sydney criteria. Additionally, we evaluated the risk of thrombotic recurrence in patients re-classified or not according to the new criteria. Methods PAPS patients meeting the Sydney criteria with previous vascular manifestations followed between 1992 and 2023 were re-evaluated to identify cases that would not be classified as PAPS based on the 2023 ACR/EULAR criteria. Results Our cohort included 205 PAPS patients: 171 met the ACR/EULAR classification criteria, 34 (16.6%) did not due to insufficient score in the laboratory domain caused by isolate IgM positivity (23 patients) or aPL positivity at a titer >99th percentile but <40 units (11 patients). Notably, 9/34 (26%) patients not confirmed as PAPS had a thrombotic relapse during follow-up. The rate of thrombotic recurrence was similar between patients who fulfilled or not the new criteria. Conclusion Among PAPS patients classified by Sydney criteria, 16.6% did not meet 2023 ACR/EULAR criteria. One quarter of them had a thrombotic relapse, confirming a pro-thrombotic profile. Inappropriately using these criteria as diagnostic in clinical practice could result in the lack of adequate antithrombotic therapy, exposing these patients to the risk of a new thrombotic event.